The present invention relates to a method for the preparation of a nickel/lanthana-comprising catalyst. Said catalyst is suitable for reforming a hydrocarbon feed to produce syngas.

Processes for reforming hydrocarbons to produce a reformed gas composition comprising synthesis gas, also called syngas, are known in the art and include steam reforming, catalytic dry reforming, partial oxidation or combinations thereof.

Steam reforming of hydrocarbons is a well known method for producing syngas and involves contacting the hydrocarbon with steam. Steam reforming is highly endothermic and requires high reaction temperatures of e.g. 700-1100°C. Accordingly, care should be taken to avoid thermodynamic constraints. Furthermore, steam reforming of hydrocarbons requires relatively long contact times. Typically, the syngas mixture produced by steam reforming of a hydrocarbon such as methane has a very high H₂/CO ratio of approximately 4.5-5.2. The H₂/CO ratio of syngas produced by steam reforming methane may be adapted e.g. by adding CO or by removing H₂. Alternatively, the H₂/CO ratio of a syngas composition may be adapted to a desired value by subjecting it to the reverse water-gas shift reaction.

A syngas composition with a H₂/CO ratio of approximately 1 can be produced directly by catalytic dry reforming of methane with CO₂. Also catalytic dry reforming of methane is highly endothermic and should be executed at high reaction temperatures. Many catalytic dry reforming processes are known to involve rapid coke deposition leading to catalyst inactivation. In these catalytic dry reforming processes the reactor can be regenerated by feeding oxygen to the catalyst under high temperatures.

Partial oxidation in the presence of a hydrocarbon feed is a further means to produce a syngas mixture. A disadvantage of partial oxidation is that carbon dioxide is produced as a by product, which limits the selectivity for aliphatic and aromatic C2-C6 hydrocarbons of the hydrocarbon reforming process.

By combining different reforming reactions including those described herein above, the reforming process can be optimized e.g. by circumventing thermodynamic constraints and/or by reducing the costs for heating or process heat removal.

A particularly advantageous combined reforming process for producing syngas is catalytic autothermal dry reforming of lower hydrocarbons such as methane. By contacting a mixture of methane (CH₄), oxygen (O₂) and carbon dioxide (CO₂) with a catalyst, endothermic dry reforming and exothermic methane oxidation can be performed in a single regime, which represents an effective means to decrease the energy consumption during syngas synthesis; see e.g. MORTOLA, et al. Eds. NORONHA, et al. Elsevier, 2007. p.409-414. A further advantage of autothermal dry reforming is that the H₂/CO ratio of the produced syngas composition is approximately 1.4-1.8, which is highly advantageous for the further use of the produced syngas in processes such as Fischer-Tropsch (F-T) synthesis.

It has been described that also nickel-based catalysts may be used in a process for the partial oxidation of light hydrocarbons to syngas. Particularly, catalysts comprising nickel supported on lanthana are known to allow a relative high conversion and selectivity in a process for producing syngas from light hydrocarbons; see e.g. EP 741107 A; US 5447705 and US 2004/0127351.

SWAAN, et al. Catalysis Today 21, 1994. P.571-578 describe a catalyst useful in a process for methane reforming that was first prepared by impregnating the support with the Ni-salt solution ([Ni(NH₃)₆]NO₃ solution), drying and calcining the catalyst precursor. Subsequently, the prepared catalyst composition is transferred to a quartz tubular reactor and is reduced in situ.

ZHAN and VERIKION J Chem Soc Chem Commun, 1995. P.71-72 describe a Ni/La203 catalyst comprising 17 wt% Ni which was prepared by the wet impregnation method, using Ni(NO₃)₂ as the metal precursor. Subsequently, the water was evaporated under continuous stirring and the residue was dried at 110°C for 24hrs. The dried residue was then ground, sieved and calcined at 500°C. Before use, the catalyst is reduced by H₂ at 500°C and 750°C so that the fresh catalyst consists of metallic Ni and La₂O₃ only.

A drawback when using a conventional Ni/La₂O₃ reforming catalyst in autothermal dry reforming of e.g. methane is deactivation of the catalyst, at least partially due to coke formation. This catalyst deactivation leads to a reduced hydrocarbon selectivity and reduced process economy.

The technical problem underlying the present invention is the provision of an improved catalyst useful in a process for converting hydrocarbons such as methane to syngas.

The solution to the above problem is achieved by providing the embodiments as described herein below and as characterized in the claims.

Accordingly, the present invention provides a method for preparing a nickel/lanthana-(Ni/La₂O₃) catalyst comprising the steps of (a) depositing nickel (Ni) on a lanthana (La₂O₃) support by contacting said La₂O₃ support with an aqueous nickel-salt (Ni-salt) solution in the presence of an oxygen-comprising gas stream; followed by (b) reducing the deposited Ni, whereby said Ni/La₂O₃ catalyst is prepared in situ; or -stated otherwise- wherein both process steps (a) and (b) are performed in the same enclosure that is later present in the process installation in which the catalysed process takes place.

Conventionally, Ni/La₂O₃ reforming catalysts are produced by chemical vapour deposition of Ni on La₂O₃ before loading the produced Ni/La₂O₃ catalyst particles into a catalyst enclosure. These previously described Ni/La₂O₃ catalysts, however, exhibit a reduced hydrocarbon conversion rate after prolonged catalyst operation and, thus, gradually become deactivated. In the context of the present invention, it was surprisingly found that a Ni/La₂O₃ reforming catalyst that is prepared in situ can be continuously operated for more than 14 days without showing a significant loss of catalyst activity. This leads to superior activity of in situ prepared Ni/La₂O₃ catalyst of the present invention after prolonged catalyst operation when compared to a conventionally prepared Ni/La₂O₃ reforming catalyst.

The term "in situ" is well known in the field of chemical engineering and refers to industrial plant operations or procedures that are performed in place. For example, aged catalysts in industrial reactors may be regenerated in place (in situ) without being removed from the reactors; see e.g. WO 03/041860 and WO 03/076074. In the context of the present invention, accordingly, a catalyst that is prepared in situ refers to a catalyst that is prepared in place, i.e. in the same enclosure that is later present in the process installation in which the catalysed process takes place.

In one embodiment, the method of the present invention is performed in such way that the Ni/La₂O₃ catalyst is prepared in a catalyst enclosure that is situated in the process installation, like a reactor for making syngas, wherein the Ni/La₂O₃ catalyst is to be employed. In a further embodiment, the method of the present invention is performed in such way that the Ni/La₂O₃ catalyst is prepared in a catalyst enclosure which can be directly placed into said process installation.

In the method of the present invention, the La₂O₃ support is preferably contacted with a Ni-salt solution in step (a) as described herein above until at least approximately 0.1 mass% Ni, preferably approximately 1 mass% Ni but no more than approximately 5 mass % Ni, preferably no more than 4 mass% Ni is deposited on the La₂O₃ support. Most preferably, approximately 3 mass% Ni is deposited on the La₂O₃ support. In the context of the present invention, the mass% of Ni load of the catalyst is calculated based on the weight of the La₂O₃ support.

The aqueous Ni-salt solution that is preferably used to deposit Ni on the La₂O₃ support in the method of the present invention is an aqueous solution of Ni(NO₃)₂. However, the skilled artisan is readily capable of selecting other suitable Ni-salts for preparing a Ni-salt solution that may be used in the method of the present invention. Such a suitable Ni-salt includes, but is not limited to, Nickel acetate.

When selecting the size of La₂O₃ support particles it should be considered that selecting a too small La₂O₃ particle size (high mesh size number) may be associated with an undesirable pressure drop in the reactor, whereas selecting a too big La₂O₃ particle size (low mesh size number) may lead to an ineffective impregnation with the Ni-salt solution. In one embodiment, the La₂O₃ support are La₂O₃ particles having a particle size of approximately 14 to 60 mesh before contacting the La₂O₃ support with the aqueous Ni-salt solution. More preferably, La₂O₃ particles of 25 to 40 mesh are used in the method of the present invention. Mesh is understood to refer to standardized mesh sizes, as defined by a.o. ASTME 11-70.

The lanthana (La₂O₃) support may further comprise a suitable inert binder such as alumina (Al₂O₃), silica (SiO₂), titania (TiO₂), zirconia (ZrO₂), magnesia (MgO) and mixtures thereof. In this case the La₂O₃ support is combined with the binder by conventional methods, e.g. MgO-La₂O₃, and then in-situ impregnated with Ni. However, it is preferred that the catalyst of the present invention does not comprise such a binder.

The Ni-salt solution is deposited on the La₂O₃ support in the presence of a gas stream comprising molecular oxygen. Without wishing to be bound by any theory, it is believed that the presence of oxygen allows the production of Ni-oxide phase and thus prevents segregation of metallic Ni-phase, which can form in the support medium when deposited without sufficient molecular oxygen present. Preferably, the La₂O₃ support is contacted with an oxygen-comprising gas stream having a temperature of approximately 450°C before contacting the La₂O₃ support with the Ni-salt solution. Preferably, the La₂O₃ support is contacted with said oxygen-comprising gas stream for at least 30 minutes and more preferably for at least 45 minutes, but not longer than 24 hours and more preferably no longer than 8 hours before contacting the La₂O₃ support with the Ni-salt solution. Most preferably, the he La₂O₃ support is contacted with said oxygen-comprising gas stream for approximately 1-2 hours before contacting the La₂O₃ support with the Ni-salt solution.

Further gaseous components comprised in the oxygen-comprising gas stream present when depositing the Ni-salt solution on the La₂O₃ may be selected from the group consisting of methane (CH₃), carbon dioxide (CO₂) and one or more inert gasses. The term "inert gas" as used herein relates to any element or compound that is in gas phase under the corresponding conditions and which does not participate in and/or interfere with the process or chemical reactions taking place during the corresponding process step. Accordingly, the "inert gas" that may be present in the respective gas stream can act as an inert diluent of the remaining gas components of the reformed gas. A preferred the inert gas in the context of the present invention is nitrogen (N₂). In one embodiment, the oxygen-comprising gas stream present when depositing the Ni-salt solution has the same composition as the hydrocarbon comprising feed stream that is subjected to autothermal dry reforming to produce syngas in the presence of the catalyst produced in the method of this invention. Even more preferably, the oxygen-comprising gas stream present when depositing the Ni consists of approximately 27-30 % mole CH₄ + 10-12 % mole O₂ + 17-18 % mole CO₂ + 41-44 % mole N₂ (the total amounting 100%), more preferably of about 28.4 % mole CH₄ + 11 % mole O₂ + 17.4 % mole CO₂ + 42.8 % mole N₂.

After depositing Ni on the La₂O₃ support until the desired Ni load is achieved, the deposited Ni is reduced in situ to its metallic state. The deposited Ni is preferably reduced by contacting the deposited Ni for 30 minutes to 24 hours with a methane-comprising gas stream having approximately 650°C-750°C. Most preferably, the deposited Ni is reduced at about 660°C.

Further gaseous components comprised in the methane-comprising gas stream used in the reducing step of the present method may be selected from the group consisting of O₂, CO₂ and inert gasses as defined herein above. In one embodiment, the methane-comprising gas stream for reducing the Ni-salt solution has the same composition as the hydrocarbon comprising feed stream that is subjected to autothermal dry reforming to produce syngas in the presence of the catalyst produced in the method of this invention. In that case, the produced Ni/La₂O₃ initially fully oxidizes methane to carbon dioxide. When reaching steady state condition, the deposited Ni is reduced and the in situ produced Ni/La₂O₃ produces syngas. Even more preferably, the methane-comprising gas stream for reducing the deposited Ni consists of approximately 27-30 % mole CH₄ + 10-12 % mole O₂ + 17-18 % mole CO₂ + 41-44 % mole N₂ (the total amounting 100%), more preferably of about 28.4 % mole CH₄ + 11 % mole O₂ + 17.4 % mole CO₂ + 42.8 % mole N₂.

In a further embodiment, the present invention relates to a process for producing syngas comprising contacting a hydrocarbon feed stream suitable for producing syngas with a catalyst produced by the method of the present invention as described herein above and as characterized in the claims. Accordingly, the present invention further relates to a process for producing syngas comprising preparing a Ni/La₂O₃ catalyst comprising (a) depositing nickel (Ni) on a La₂O₃ support by contacting said La₂O₃ support with an aqueous Ni-salt solution in the presence of an oxygen-comprising gas stream; followed by (b) reducing the deposited Ni, whereby said Ni/La₂O₃ catalyst is prepared in situ and subsequently contacting said prepared catalyst with a hydrocarbon feed stream suitable for producing syngas.

The hydrocarbons comprised in the hydrocarbon feed may include gaseous hydrocarbons such as methane, ethane, natural gas or liquefied petroleum gas (LPG) but also more heavy hydrocarbons such as C6+ hydrocarbons. Preferably, the main hydrocarbon constituent comprised in the hydrocarbon feed of the process of the present invention is methane. In the context of the present invention, the term "main hydrocarbon constituent" means that a given hydrocarbon constitutes more than 50 % mole of all comprised hydrocarbons, more preferably more than 60 % mole of all comprised hydrocarbons, even more preferably more than 70 % mole of all comprised hydrocarbons, and most preferably of more than 75 % mole of all comprised hydrocarbons. Accordingly, the present invention particularly preferably relates to a process for the catalytic autothermal dry reforming of methane (ATDRM) which comprises contacting the Ni/La₂O₃ catalyst produced in situ as described herein above with a methane-comprising hydrocarbon feed stream.

Further gaseous components comprised in the hydrocarbon feed as used in the reforming process of the present invention may be selected from the group consisting of O₂, CO₂ and one or more inert gasses as defined herein. In one embodiment the hydrocarbon comprising feed stream that is contacted with the catalyst produced in the method of this invention has the same composition as the oxygen-comprising gas stream present when depositing the Ni and/or the methane-comprising gas stream for reducing the Ni-salt solution when in situ preparing the catalyst of the present invention. Even more preferably, the hydrocarbon feed stream of the reforming process of the present invention consists of approximately 27-30 % mole CH₄ + 10-12 % mole O₂ + 17-18 % mole CO₂ + 41-44 % mole N₂ (the total amounting 100%), more preferably of about 28.4 % mole CH₄ + 11 % mole O₂ + 17.4 % mole CO₂ + 42.8 % mole N₂. It is evident that this composition may be changed or adapted under certain circumstances. Yet, a methane/O₂ ratio of more than about 3 should be avoided since a too low oxygen content leads to catalyst deactivation due to coke formation. At methane/O₂ ratio of less than 2.5 is also less favourable due to catalyst over-oxidation (too many oxide phases in catalyst).

It is well within the scope of the skilled artisan to determine suitable process conditions for performing a process for producing syngas comprising contacting a suitable hydrocarbon feed stream with a catalyst produced by the method of the present invention. Preferably, the (reforming) process of the present invention is performed at a reaction temperature of 650°C to 750°C and a contact time of 0.4 to 2 seconds. More preferred are a reaction temperature of 680°C to 710°C and a contact time of 0.4 to 1 seconds. At a reaction temperature of less than 680°C the conversion may start to decrease undesirably.

The term "reformed gas" as used herein relates to the gaseous product of the reforming process as described herein and essentially consists of syngas (H₂ and CO), oxygen (O₂) and optionally a further component selected from the group consisting of methane (CH₄), carbon dioxide (CO₂) and inert gas.

The main constituent of the "reformed gas", accordingly, is synthesis gas (syngas; a mixture of H₂ and CO). The reformed gas of the present invention comprises at least 50 % mole and preferably at least 55 % mole syngas. Preferably, the syngas present in the reformed gas produced by the reforming process of the present invention has a hydrogen (H₂) to carbon monoxide (CO) ratio of at least 1, more preferably of at least 1.2, even more preferably of at least 1.4 and particularly preferably of at least 1.6, but preferably of not more than 4, more preferably of not more than 3, even more preferably of not more than 2 and particularly preferably of not more than 1.8. Most preferably, the "reformed gas" has a H₂ to CO ratio of at least 1.6 but not more than 1.8 (H₂ to CO ratio of 1.6-1.8).

The "reformed gas" of the present invention may comprise at least 0.1 % mole O₂, more preferably at least 0.2 % mole O₂, even more preferably at least 0.3 % mole O₂ and particularly preferably at least 0.35 % mole O₂. Furthermore, the "reformed gas" of the present invention comprises not more than 5 % mole O₂, more preferably not more than 2.5 % mole O₂ and even more preferably not more than 1 % mole O₂ and particularly preferably not more than 0.5 % mole-O₂. Most preferably, the reformed gas made in the process of the present invention comprises 0.4 % mole oxygen (O₂).

As described herein below, a feedstream comprising CH₄, O₂ and CO₂ may be converted by catalytic dry reforming by contacting said feedstream with an in situ prepared Ni/La₂O₃ catalyst at a reaction temperature 710°C to produce a reformed gas that approximately comprises 22 % mole CO, 35 % mole H₂ and 0.4 % mole O₂.

The "reformed gas" produced by reforming hydrocarbons may be directly used as a feed for further processes including, but not limited to, F-T synthesis or carbonylation reactions like alkene hydrocarbonylation or aromatic hydrocarbons hydrocarbonylation to produce aromatic aldehydes.

F-T synthesis is a well known catalyzed chemical reaction in which synthesis gas, a mixture of carbon monoxide and hydrogen, is converted into a complex mixture of hydrocarbon compounds by contacting syngas with an F-T catalyst under F-T synthesis conditions; see e.g. KANEKO TAKAO, et al. Ullmann's Encyclopedia of Industrial Chemistry. 7th edition. John Wiley, 1997. p.17-31.

The most common catalysts useful in F-T synthesis ("F-T catalysts") are based on Fe and/or Co, although Ni- and Ru-based catalysts have also been described; see e.g. US 4,177,203; COMMEREUC, et al. J. Chem. Soc., Chem. Commun. 1980, p. 154 - 155; OKUHARA, et al. J. Chem. Soc., Chem. Commun. 1981, p.1114-1115. Generally, Ni-based catalysts are relatively more selective for producing methane whereas Co-, Fe- and Ru-based catalysts are more selective for hydrocarbons having at least one carbon atom (C2+ hydrocarbons). Moreover, the selectivity for C2+ hydrocarbons can be increased by decreasing the H₂/CO ratio, decreasing the reaction temperature and decreasing the reactor pressure.

A suitable Fischer-Tropsch catalyst that may be used is a cobalt/iron bi-component oxide (Co/Fe-O) catalyst. The cobalt/iron bi-component oxide may be supported on an inert support. Suitable inert supports for Fischer-Tropsch catalysts are known in the art and include alumina (Al₂O₃), silica (SiO₂), titania (TiO₂), zirconia (ZrO₂) and magnesia (MgO) and mixtures thereof. Preferably the cobalt/iron bi-component oxide is supported on silica (SiO₂). Such a Co/Fe-O-SiO₂ catalyst is described in YONGHONG BI, et al. The Canadian Journal of Chemical Engineering 2003, vol.81, p.230-242.

In yet another embodiment, the present invention relates to Ni/La₂O₃ catalyst produced by the method of the present invention as described herein above and as characterized in the claims. The Ni/La₂O₃ catalyst provided by the present invention is characterized by its increased stability which allows continuous operation of the catalyst in the reforming process described herein above and as characterized in the claims for more than 14 days without showing a significant loss of catalyst activity.

The present invention will now be more fully described by the following nonlimiting Examples.

The reforming catalyst "3%Ni/La₂O₃ in situ" for autothermal dry reforming of methane was prepared as follows: 2 ml La₂O₃ particles of 25-40 mesh size were loaded in the quartz reactor with an inner diameter of 12 mm and a length of 45 cm. The loaded reactor was fed with a gaseous mixture consisting of approximately 28.4 % mole CH₄ + 11 % mole O₂ + 17.4 % mole CO₂ + 42.8 % mole N₂ at 450°C. After treatment of the support with reaction mixture (less than 24 hours) to obtain a stable phase composition, impregnation of the La₂O₃ support with a 0.1-0.3% Ni(NO₃)₂ solution in water was started. Therefore the 0.1-0.3% Ni(NO₃)₂ solution was fed to the La₂O₃ support by injecting into the gas stream to gradually impregnate the La₂O₃ support with the Ni salt. The Ni(NO₃)₂ impregnation of the La₂O₃ support was continued until the calculated amount of 3 mass% Ni /La₂O₃ basis was reached. Formation of NO₂ was observed during he Ni(NO₃)₂ impregnation step which indicates that oxides of Ni are formed within the La₂O₃ support.

After completing the Ni(NO₃)₂ impregnation step, the reaction temperature was increased to 660°C to reduce the Ni-oxide with the methane containing gaseous mixture (as above). The resultant in situ prepared reforming catalyst "3%Ni/La₂O₃ in situ" initially fully oxidizes methane to CO₂. However after reaching steady state condition the "3%Ni/La₂O₃ in situ" catalyst forms CO and H₂.

Contacting the methane containing feed stream with the reforming catalyst "3%Ni/La₂O₃ in situ" was continued at 680-710°C with a contact time of 0.5 sec to produce a syngas composition consisting of approximately 21.5 % mole CO + 35.5 % mole H₂ + 34.79 % mole N₂ + 6.17 % mole CH₄ + 1.55 % mole CO₂ + 0.4 % mole O₂. A quartz reactor with an inner diameter of 12 mm located in an electrically heated furnace with 45 cm length was used. As shown in Table 1 provided herein below, the in situ prepared Ni /La₂O₃ catalyst showed stable activity for more than 120 hours.

Conventional Ni/La₂O₃ catalysts are prepared by impregnation of La₂O₃ powder with a Ni(NO₃)₂ solution at 70-80°C, to result in a Ni content of 3 mass% (as in Ex. 1). Subsequently, the Ni impregnated La₂O₃ is kept at 70-80°C until all water has evaporated. The solid mixture is then dried at 120°C and is calcined at 700°C for 4 hours. Subsequently, the calcined Ni/La₂O₃ particles are loaded into in a quartz reactor with an inner diameter of 12 mm which functioned as catalyst enclosure.

A feed stream consisting of approximately 28.4 % mole CH₄ + 11 % mole O₂ + 17.4 % mole CO₂ + 42.8 % mole N₂ was contacted with 0.5 ml the conventionally prepared Ni/La₂O₃ reforming catalyst at 680°C-710°C with a contact time of 0.5 sec to produce a syngas composition.

As shown in Table 1 provided herein below, the activity of a reforming catalyst prepared by impregnation of La₂O₃ with Ni salt gradually declined within 14 days of operation. CH₄ conversion (%) Catalyst screening duration (days) In situ prepared catalyst (Example 1) Conventionally prepared catalyst (Experiment 2) 5 73.7 72.5 14 73.0 65.8

As can be derived from Table 1, the in situ prepared catalyst can be continuously operated for more than 14 days without showing a significant loss of catalyst activity. This leads to superior activity of the catalyst of the present invention after prolonged catalyst operation, as can be derived from the significantly higher methane conversion rate after 14 days of catalyst operation when compared to a conventional catalyst prepared by chemical vapour deposition of Ni on La₂O₃.